Computer Aids for VLSI Design

Steven M. Rubin

Copyright © 1994

Chapter 11: Electric

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11.6 Designing a Chip

To illustrate the use of Electric, this section will describe the construction of a static-memory chip [Lyon and Schediwy]. This chip makes use of a novel four-transistor bit of static memory. The fundamental memory cell is logically designed using the schematics environment (see Fig. 11.12). An equivalent CMOS layout is then produced without concern for compact spacing (see Fig. 11.13). For proper layout efficiency, alternate bits of memory are different, so two bits define the leaf cell of the design. These bits can be compacted by the one-dimensional compacter (see Fig. 11.14) and then compacted further by rearranging components and recompacting (see Fig. 11.15).

Fig 11.12
FIGURE 11.12 Schematic for four-transistor static-memory cell.
Fig 11.13
FIGURE 11.13 CMOS layout for four-transistor static-memory cell.

Fig 11.14
FIGURE 11.14 CMOS layout for two four-transistor static-memory cells.
Fig 11.15
FIGURE 11.15 Compacted CMOS layout for two four-transistor static-memory cells.

To create a 128 × 32-bit array of memory, six levels of hierarchy are employed to build a 4 × 2 array, a 4 × 4 array, a 16 × 8 array, a 64 × 8 array, a 64 × 32 array, and a 128 × 32 array (see Fig. 11.16). Note that each level of hierarchy actually connects its subcells with little stitches so that the overall connectivity is maintained. These stitches are automatically created by the router. Also, each level of hierarchy must export all unstitched ports to the next level. This is done automatically by the array-based layout commands.
Fig 11.16
FIGURE 11.16 Hierarchical organization for 128 × 32 array of static-memory cells.

Once the basic memory array is created, driving circuitry must be placed on the edges. A word (32-bit) driver for a single word is designed to pitch match the memory (see Fig. 11.17) and the driver is arrayed using hierarchy. The block of 128 drivers attaches to the bottom of the memory array. Similar drivers and decoders are built on the sides. The overall floor-plan, including pads, is shown in Fig. 11.18. This layout contains 32,650 transistors, described with 110 cells.
Fig 11.17
FIGURE 11.17 Word line driver for static-memory array.

As an indication of the complexity of Electric, this circuit consumes 1.4 megabytes of disk space. Although this is not small, it does contain more information than typical design databases. It can be read in 50 seconds and written in 30 seconds (on a SUN/3 workstation). Netlist generation takes only 15 seconds (these figures were compiled in 1986, when computers were significantly slower).
Fig 11.18
FIGURE 11.18 Floor-plan of static-memory chip.

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Steven M. Rubin
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